1. Field of the Invention
This invention relates to optical printing systems and more particularly to a high speed optical printing system employing a two dimensional holographic array, a hologram element of which is accessed by a spatially unmodulated substantially collimated light beam and in response thereto emits a spatially modulated beam which forms an image on a photoconductive drum.
2. Description of the Prior Art
Standard impact printer systems employ a rotating or translating member that contains one or more sets of the character vocabulary. To access the proper character an instant of time is selected, from the total available cycle time, at which the impact of a hammer transfers the desired character from the rotating of translating member to the printout paper. This approach has been employed to produce an optical printer wherein a light beam is directed, at the proper time, to a mask containing transparencies or templates of the character vocabulary to illuminate the desired character contained therein. The spatially modulated beam resulting therefrom is focussed upon a photoconductive drum member of an electrophotographic copier system wherein the character is rendered visible and transferred to a hard copy paper image. This time domain accessing of characters requires the light beam, which illuminates the rotating character mask, to be pulsed not only at the proper time to select the desired character but in such a fashion as to hold character blurring within acceptable tolerances. Consequently, the average light pulse duty cycle must include a pulse that is sufficiently brief to provide an instantaneous snapshot of the selected character at an average interpulse period that is equivalent to one cycle of the entire vocabulary of characters. To prevent blurring, the character motion must be less than five percent of the character dimension. Thus, if the printable vocabulary contains 100 symbols, an average light pulse duty cycle of 5.times.10.sup.-4 is required. This duty cycle dictates a peak power level for the pulse system's light source that is 2,000 times greater than that of a light source in an illumination system which could function with a 100 percent duty cycle.
Time domain accessing for optical printing systems as described above exhibit printing speed limitations. A system employing a character reel that rotates at 3600 rpm and carries two vocabulary sets on its circumference has a printing rate of 120 characters per second or approximately 60 lines of printing per minute. To surmount this printing speed limitation a multiplicity of light sources has been employed and printing speeds of several thousand lines per minute have been achieved by employing one light source for each printing line on a page. This is a brute force approach that is expensive and which results in a short mean time between failures (MTBF). It will readily be appreciated that printing speeds of optical printing systems may be increased, without character blurring, by replacing the time dimension accessing procedure with a system that randomly positions a beam to access a stationary array of character generating masks.